The present invention relates generally to rapid solidification techniques for producing metallic powders, and more particularly to method and system for large scale production of contamination free powder of high melting temperature reactive and nonreactive metals and alloys.
In industrial applications of metal and alloy powders, spherical powders which flow well and have consistently high tap density are specially desirable in powder metallurgy processes for consolidation by way of vacuum hot pressing at high pressure to pressed parts with near net product shape. Metallic powders produced by rapid solidification of molten droplets of the constituent metal or alloy may generally have ultrafine grain structure and are therefore particularly desirable for finished pressed parts. Rapid solidification may also be used to supersaturate powder of a metal host with a selected alloying constituent which upon heat treatment of a pressed part results in useful alloy phases not obtainable by conventional heat treatment methods. In the fabrication of powder from alloys useful in fabricating aerospace components, contaminants in the powder must be carefully controlled and excluded since even small contaminant levels may have substantial deleterious effects on metallurgical and physical properties of the pressed powder which may result in severe degradation or fatal defects in a finished component.
Conventional methods for producing metallic powder include chemical methods wherein powder is produced by chemical decomposition of a metal compound, mechanical methods wherein the metal form is mechanically comminuted to the desired particle size, and physical methods wherein a molten stream of the metal or alloy is atomized by impact with a fluid, usually gas, jet. Atomization processes are commonly used in the production of metallic powders, and are the most convenient for production of alloy powders of the type required for modern high temperature applications. Such an atomization process is generally a two step process comprising providing a melt of the metal or alloy, followed by disintegrating a molten stream of the melt into droplets by impact with one or more high pressure fluid streams. Powders in the size range of from about 0.1 to about 1000 microns may be produced. In the production of rapidly solidified metallic powder utilizing gas atomization techniques, small particles solidify faster and often into a different microstructure than large particles; accordingly, microstructural uniformity in a finished powder compact requires close control of particle size in limited size ranges. Atomization processes may be applicable to the production of powders of most metal and alloys of interest including iron, tin, nickel, copper, aluminum, titanium, tungsten, molybdenum, tantalum, niobium, magnesium and the alloys including stainless steels, bronze, brass and nickel/cobalt based superalloys. A comprehensive survey of conventional atomization techniques is presented in "Production of Rapidly Solidified Metals and Alloys", by S. J. Savage and F. H. Froes, J Metals 36:4, 20-33 (April 1984).
High purity powders of reactive and/or high melting point metals or alloys are difficult to produce utilizing presently known atomization processes, mainly because a shortcoming exists in those processes in that melting and pouring of molten metallic material in a controlled flow through the die or other atomization means using furnaces, crucibles, nozzles or other handling means contribute contaminants to the melt. Reactive and/or high melting melts rapidly erode process equipment resulting in abbreviated production runs, high maintenance costs and contamination of the powder product. Certain existing processes which do not use a nozzle to confine or to direct the melt (see, e.g., rotating electrode process and others described in Savage et al, supra) are incapable of producing powder in a desirably fine size range or at acceptable production rate.
The present invention provides system and method for producing substantially contamination free powder of reactive, nonreactive and high melting metals and alloys which comprises atomization of a molten stream directed from a melt suspended electromagnetically. In the method of the invention, electromagnetic induction means conventionally used for melting and levitation is combined uniquely with electromagnetic confinement means for controlling the shape and flow rate of a molten metal stream and an atomization process for disintegrating the stream into droplets for solidification into powder without direct contact of the melt, molten stream or molten droplets with the process equipment. Levitation melting in vacuum or inert gas without a conventional nozzle according to the invention substantially eliminates contamination of the molten stream. The method and system of the invention therefore provides large scale production of powder at solidification rates substantially higher than that of conventional processes. A wide alloying range for metallic powder product is achievable, and, compared to previously known systems, substantial savings in equipment maintenance cost is realized.
It is therefore a principal object of the invention to provide method and system for producing rapidly solidified metallic powder.
It is a further object of the invention to provide method and system for producing contamination free metallic powder.
It is another object of the invention to provide method and system for large scale production of metallic powder.
These and other objects of the invention will become apparent as the description of representative embodiments proceeds.